The invention relates to a device for preventing the creeping of an optical element, in particular a lens or a mirror, the optical element being connected to a mount via connecting members arranged on the circumference of the optical element, and the position of the optical element in an objective deviating from the vertical axial position.
To date, in semiconductor lithography, optical elements have been held in a mount by means of various clamping techniques, clamping in combination with self-closure and via bonded connections, for example by gluing. It is generally known that in the case of screwed connections it is necessary to shape the screws so as to keep the elasticity of the screw shank as low as possible in order to keep within a tolerable range the loss of prestressing force owing to setting and relaxation effects of the shaft. Elements of high elasticity are used with clamped connections or mechanical coupling points in order to thus minimize the effects of tolerances during installation, and to minimize temporal changes acting during operation on the parts that determine functioning.
It is also known from the older DE 102 11 791.8 that in conjunction with a vertical optical axis the optical element is glued onto elastic connecting members or spring elements. Before the optical element is glued in, it is laid onto the elastic connecting members, and the optical axis of the optical element is aligned parallel to the mount axis. In this case, gravity acts approximately along the optical axis of the optical element. The optical element can then be connected or glued to the elastic connecting members.
It is possible for a mount to be arranged in projection objectives together with the optical element such that the optical axis of the optical element is horizontal or is inclined at a specific angle to the vertical. As a result, gravity acts not only along the optical axis of the optical element, but also transverse thereto. Because of the bearing of the optical element on the elastic connecting members, the effect of gravity acting transverse to the optical axis is generally a displacement of the optical axis of the optical element transverse to the mount axis and, possibly, a tilting of the optical axis of the optical element in relation to the mount axis. The elastic connecting members are deformed by the weight force, acting transverse to the optical axis, of the optical element in such a way that the optical element is displaced in the lateral direction in relation to its original position and is tilted.
A lateral displacement of the optical element can be corrected during installation in the projection objective by means of an appropriate displacement of the mount, no tilting correction being possible by displacing the mount during installation of the objective. As is known, for example, from Stuart. T-Smith: Flexures; Gordon and Breach Science Publishers, 2000, it is possible to use the customary methods of elasticity theory in order to shape and design the elastic connecting members such that they lead not to a tilting of the optical element, but only to a lateral displacement in the event of loading by the force of gravity on the optical element transverse to the optical axis. The position of the centroid of the optical element plays an important role in the design of the elastic connecting members to counteract tilting of the optical element. The sectional loads, resulting in the event of lateral offset of the optical element without tilting, of the elastic connecting members between connecting members and optical element should, when combined as resulting force, form a force through the centroid of the optical element that is of the same magnitude as gravity and acts against it. Since, however, the elasticity of the connecting members can fluctuate owing to manufacturing tolerances, a displacement of the resulting force, and thus an impossibly large tilting of the optical element can occur despite a theoretically correct design of the connecting members counter to tilting. The abovementioned correction of the lateral offset is, however, possible only when the offset does not change with time. In a preferred fastening of the optical element by means of gluing, however, it is possible for the glue to creep as a consequence of shear stresses, and thus, again, for there to be a temporally variable lateral offset.
It is likewise known from the older DE 102 11 791.8 for there to be inserted between the elastic connecting members and the optical element inserts such as, for example, angles or wedges, that provide additional gluing points between the optical element and the connecting members via suitable connections. Even by thus lowering the glue stress and raising the stability, the load changes caused by relaxation, setting and creep phenomena of the optical element at the connecting points cannot fundamentally be reduced. The inserts only put off the problem. Likewise, no glues free from creep are yet known.
Lateral bearings for the optical element via two “half” bearings are likewise known from Paul R. Yoder: Design and Mounting of Precision and Small Mirrors in Optical Instruments, Spie Volume TT32, page 154. With any type of fastening, for example screwing an optical element on or gluing it, large local defects arise here in the region of the fastening points of the optical element. However, as is known from U.S. Pat. No. 4,733,945, a simple mounting technique is not possible by spherization.
It is likewise known from investigations relating to lateral bearing of large mirrors to permit the lateral bearings to act tangentially or at a specific angle to the tangent, and thereby to minimize the bending of the optical element as far as possible. Such investigations are described in the Journal of modern Optics, 1988, by Schwesinger, G.: Lateral support of very large telescope mirrors by edge forces only. If the tangential bearings are glued onto the optical element, the lens likewise creeps through being acted upon by gravity.
Fastening the optical element by soldering, as is known from DE 197 35 760 A1, produces no advantageous improvement, since the optical element is distorted or warped when experiencing thermal expansion. Fastening by clamping produces local stresses and requires a larger optically unused overflow of the optical element.
Again, active bearings known, for example, from DE 100 51 706 A1, such as readjusting the creeping movement of the optical element with the aid of an XY manipulator and/or a tilt manipulator constitute only unsatisfactory solutions. It is true that the rigid body movement would be compensated in the case of such solutions, but not so the deformations, produced by stress redistribution at the gluing points, of the optical element. Moreover, such bearings require a complicated design that entails high costs and requires electronic control.
Reference may be made to Hale, L. et al.: New photolithography stepping machine, Lawrence Livermore National Laboratory, 1995, UCRL-ID-120313, page 4 ff. and page 25 for the further prior art.
It is therefore an object of the invention to provide a device for preventing creeping of optical elements arranged and glued in mounts, optical axes of the optical elements not being vertical.